1. Technical Field
The present invention relates to a controlling method and a controlling device of an optical disc drive, and more particularly to a spherical aberration compensation method and a spherical aberration compensation device of an optical disc drive.
2. Description of the Related Art
FIG. 1 is a schematic diagram illustrating a spherical aberration effect observed in a lens. In a case that the spherical aberration occurs, the light beams passing through the paraxial zone A1 of the lens 120 are focused on the point A on a plane 100, but the light beams passing through the axial zone B1 of the lens 120 are focused on the point B in front of the plane 100. That is, since the focus points of the light beams passing through the axial zone and the paraxial zone are not superimposed with each other, the spherical aberration occurs. The spherical aberration is usually abbreviated as “SA”.
Generally, the thickness deviation of a cover layer (also referred as a transparent plastic layer) is a principal reason that causes the spherical aberration. Once the spherical aberration occurs, the quality of the playback signal such as a radio frequency signal (RF) or a servo signal (e.g. a focusing error signal (FE) or a tracking error signal (TE)) will be deteriorated to adversely affect the performance of reproducing the optical disc.
For reducing the spherical aberration, a spherical aberration compensator is usually installed in an optical disc drive. In addition, a spherical aberration calibration (SA calibration) is added to a start-up procedure of the optical disc drive. In such way, after a spherical aberration compensation value is received by the spherical aberration compensator, the influence of the spherical aberration will be decreased. That is, the focuses of the light beams passing through the axial zone and the paraxial zone will be substantially at the same position. After the start-up procedure of the optical disc drive is implemented, normal data access of the optical disc will be started.
Generally, the common spherical aberration compensators are classified into two types, i.e. an LCD-type spherical aberration compensator and a collimating-type spherical aberration compensator. When a spherical aberration compensation value is received by the LCD-type spherical aberration compensator, the refractive index thereof is changed to reduce the optical path difference between the paraxial zone and the axial zone is reduced. As such, the influence of the spherical aberration is decreased. On the other hand, when a spherical aberration compensation value is received by the collimating-type spherical aberration compensator, a relative position of an optical element thereof is adjusted to reduce the optical path difference between the paraxial zone and the axial zone is reduced. As such, the influence of the spherical aberration is decreased.
As known, for performing the start-up procedure, the optical pickup head of the optical disc drive is controlled to move to an inner track position of the optical disc. In other words, the SA calibration is performed at the cover surface of the optical disc near the inner track position. The spherical aberration compensation value acquired from the SA calibration is only effective to reduce the spherical aberration at the inner track region. If the thickness of the cover layer of the optical disc is uniform, the spherical aberration compensation value is effective to reduce the spherical aberration all over the optical disc.
The thickness of the cover layer of the optical disc, however, might be non-uniform from the inner track to the outer track due to process variation. Under this circumstance, when a SA calibration is performed at the inner track region to acquire the spherical aberration compensation value (also referred as a SA value), the spherical aberration compensation value is not effective to reduce the spherical aberration at other region (e.g. an outer track region) of the optical disc. As such, the playback signal read from the outer track region of the optical disc has deteriorated quality. The situation is worse when the optical pickup head fails to successfully focus on the outer track region of the optical disc or the playback signal fails to be generated.
FIGS. 2A and 2B are schematic diagrams illustrating two kinds of optical discs with non-uniform cover layer thickness. As shown in FIG. 2A, the optical disc 200 has a central hole 205 in the middle thereof. The thickness of the cover layer 220 of the optical disc 200 is non-uniform. From the surface of the cover layer 220 to the data layer 210 of the optical disc 200, the inner track region is relatively thinner but the outer track region is relatively thicker. As shown in FIG. 2B, the optical disc 250 has a central hole 255 in the middle thereof. The thickness of the cover layer 270 of the optical disc 250 is also non-uniform. From the surface of the cover layer 270 to the data layer 260 of the optical disc 250, the inner track region is relatively thicker but the outer track region is relatively thinner.
For compensating the spherical aberration resulting from the non-uniform thickness of the cover layer of the optical disc, a spherical aberration compensation method is disclosed in for example Taiwanese Patent Publication No. 200929197 (corresponding to US Patent Publication No. 20090168616), which is entitled “Spherical aberration compensation method of optical storage device”.
FIG. 3 is a flowchart illustrating a spherical aberration compensation method disclosed in US Patent Publication No. 20090168616. After the optical disc drive is activated (Step 300), the optical pickup head is moved to a first track position (Step 302), and a spherical aberration calibration is performed at the first track position to acquire a first reference value (Step 304). Then, the optical pickup head is moved to a second track position (Step 306), and a spherical aberration calibration is performed at the second track position to acquire a second reference value (Step 308). Then, normal data access of the optical disc is started (Step 310). According to the first and second reference values, an interpolation is performed to acquire an interpolated spherical aberration compensation value (Step 312).
Then, check whether the interpolated spherical aberration compensation value is different from a current spherical aberration compensation value (Step 314). If these two values are different from each other, the interpolated spherical aberration compensation value is utilized to update the current spherical aberration compensation value (Step 316), and then go to Step 312. Otherwise, if these two values are identical, go to Step 312, and the current spherical aberration compensation value is not updated.
That is, according to the conventional spherical aberration compensation method, a first spherical aberration compensation value (i.e. a first reference value) is acquired by performing a spherical aberration compensating correction (i.e. spherical aberration calibration) at the inner track region of the optical disc, and then a second spherical aberration compensation value (i.e. a second reference value) is acquired by performing a spherical aberration compensating correction (i.e. spherical aberration calibration) at the outer track region of the optical disc. According to the first and second reference values, an interpolation is performed to obtain an interpolated spherical aberration compensation value at any radius position of the optical disc. When the optical pickup head is at any position of the optical disc, the interpolated spherical aberration compensation value is inputted into the spherical aberration compensator to reduce the influence of the spherical aberration.
As known, the process of performing the spherical aberration calibration is time-consuming. Since the conventional spherical aberration compensation method needs two spherical aberration calibration steps, it takes more time to perform the start-up procedure. In other words, the timing of accessing the data of the optical disc is delayed.
Moreover, in a case that the optical disc has two data layers, a first spherical aberration calibration and a second spherical aberration calibration are respectively performed on the first and second data layers of the optical disc at the inner track region to acquire first and second spherical aberration compensation values corresponding to the first and second data layers of the optical disc at the inner track region. Then, a third spherical aberration calibration and a fourth spherical aberration calibration are respectively performed on the third and fourth data layers of the optical disc at the outer track region to acquire third and fourth spherical aberration compensation values corresponding to the first and second data layers of the optical disc at the outer track region. According to the first, second, third and fourth spherical aberration compensation values, a first interpolated spherical aberration compensation value corresponding to the first data layer and a second interpolated spherical aberration compensation value corresponding to the second data layer are obtained.
Since the conventional spherical aberration compensation method of the optical disc having two data layers needs four spherical aberration calibration steps, it takes more time to perform the start-up procedure. In other words, the timing of accessing the data of the optical disc is largely delayed.